Interesting things going on over at
Ryandor's forums - a client hack has been found for customizing the map-sizes that the client expects of the various map# files. In theory, this means that it could be possible to have mapsizes of for instance 10000x10000 for both map0, map2, map3 and map4. Or what about a map sized 30000x30000?

Of course, a map like that would be horribly big, and there are probably no mapeditors actually capable of handling it, but... no more restricted map-sizes anyway
They're also working on a patch/program to handle the hack, so people don't have to hex-edit their clients manually.
EDIT: You can also edit the "wrap point" of the map, i.e. the black strip down the map which can be used to determine where the world "wraps around", sort of.